Control Structures. March 1, Dr. Mihail. (Dr. Mihail) Control March 1, / 28

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1 Control Structures Dr. Mihail March 1, 2015 (Dr. Mihail) Control March 1, / 28

2 Overview So far in this course, MATLAB programs consisted of a ordered sequence of mathematical operations, functions, etc. In other words, these programs have had a sequential structure. We will learn two other structures: selection structures repetition (loop) structures (Dr. Mihail) Control March 1, / 28

3 Selection and Repetition Definition: a selection structure allows execution of a set of commands when some selection criteria is true, or a second set of commands when the selection criteria is false. The selection criteria consists of one or more logical conditions that can evaluate to EITHER true or false (never both). The evaluation of these conditions often involves the use of relational and logical operators. Definition: a repetition structure (or loop) allows repeated (zero, one or more times) execution of a set of commands. The number of times a loop is executed can depend on: a counter OR result of a logical condition (Dr. Mihail) Control March 1, / 28

4 Selection and Repetition (Dr. Mihail) Control March 1, / 28

5 Selection and Repetition Relational Operators Definition: relational operators define relations between two entities of the same size (matrices, vectors and scalars). Relations include numerical equality and inequality. (Dr. Mihail) Control March 1, / 28

6 Selection and Repetition Relational Operators Definition: relational operators define relations between two entities of the same size (matrices, vectors and scalars). Relations include numerical equality and inequality. (Dr. Mihail) Control March 1, / 28

7 Selection and Repetition Relational Operators The result of equality or inequality tests are either true (numeric value of 1) of false (numeric value of 0). This data type is called boolean. Example: >> a = [ ]; >> b = [ ]; >> a>b ans = >> a==b ans = (Dr. Mihail) Control March 1, / 28

8 Selection and Repetition Logical Operators One can combine boolean values with logical operators: (Dr. Mihail) Control March 1, / 28

9 Selection and Repetition Examples in Class Please take notes! (Dr. Mihail) Control March 1, / 28

10 Truth Tables AND (&) A B ( A & B ) F F F F F F F F F F F (Dr. Mihail) Control March 1, / 28

11 Truth Tables OR ( ) A B ( A B ) F F F F F F F F F (Dr. Mihail) Control March 1, / 28

12 Truth Tables NOT A A F F F (Dr. Mihail) Control March 1, / 28

13 Truth Tables Expression: (A & ( B C)) A B C ( A & ( B C ) ) F F F F F F F F F F F F F F F F F F F F F F F F F F F F F F F F F F F (Dr. Mihail) Control March 1, / 28

14 Operator Precedence 1 Parentheses () 2 Transpose, power, complex conjugate transpose, matrix power 3 Unary plus, unary minus, logical negation 4 Multiplication (.*), division (./), matrix multiplication (*), matrix division (/) 5 Addition (+), subtraction (-) 6 Colon operator (:) 7 Less than, less than or equal to, greater than, greater than or equal to, equal to, not equal to 8 Element-wise AND 9 Element-wise OR (Dr. Mihail) Control March 1, / 28

15 Operator Precedence MATLAB always gives the & operator precedence over the operator. Although MATLAB typically evaluates expressions from left to right, the expression a b&c is evaluated as a (b&c). It is a good idea to use parentheses to explicitly specify the intended precedence of statements containing combinations of & and. (Dr. Mihail) Control March 1, / 28

16 Selection Structures Two selection structures: find command family of if structures (Dr. Mihail) Control March 1, / 28

17 find find(x) returns a vector composed of the indices of the nonzero elements of x. These can be used in further commands, and often eliminates the need for if or loop structures. Trivial example: >> x = [ ]; >> find(x) ans = (Dr. Mihail) Control March 1, / 28

18 find find(x) returns a vector composed of the indices of the nonzero elements of x. These can be used in further commands, and often eliminates the need for if or loop structures. Another example: >> x = [ ]; >> y = (x==0); >> find(y) ans = 4 5 (Dr. Mihail) Control March 1, / 28

19 find >> A = magic(8) A = (Dr. Mihail) Control March 1, / 28

20 find >> A = magic(8); >> indices = find(a>50) indices = (Dr. Mihail) Control March 1, / 28

21 Linear indices Linear indices. Example: 10 by 3 matrix (Dr. Mihail) Control March 1, / 28

22 Linear indices >> A = magic(3) A = >> A(:) ans = (Dr. Mihail) Control March 1, / 28

23 one-way if statements Syntax if condition statements end Example: x = input( Please enter positive x: ); if(x<0) fprintf( You have entered a negative number!\n ); end (Dr. Mihail) Control March 1, / 28

24 two-way if statements Syntax if condition statements when condition is true else statements when condition is false end Example: x = input( Please enter x between 10 and 20 : ); if( (x>=10)&(x<=20) ) fprintf( Good input!\n ); else fprintf( Bad input!\n ); end (Dr. Mihail) Control March 1, / 28

25 multi-way if statements Syntax if condition statements when condition is true elseif another_condition statements when another_condition is true elseif yet_another_condition statements when yet_another_condition is true else statements when all conditions above are false end (Dr. Mihail) Control March 1, / 28

26 multi-way if statements Example if temerature>100 disp( Too hot-equipment malfunctioning ); elseif temperature>90 disp( Normal operating temperature ); elseif temperature>50 disp( Temperature below desired operating range ); else disp( Too cold-turn off equipment ); end (Dr. Mihail) Control March 1, / 28

27 while loops Syntax while condition statements when condition is true end Example: counting with a counter c = 1; while(c<=10) fprintf( %d\n, c); c = c + 1; end (Dr. Mihail) Control March 1, / 28

28 for loops Syntax for index = sequence body end Example: evaluate s = n for n = 10: n = 10; s = 0; for i = 1:n s = s + i; end fprintf( s = %d, s); (Dr. Mihail) Control March 1, / 28